The invention provides a process for the extraction of hydrogen from a gas mixture comprising at least hydrogen and carbon monoxide and optionally one or more other gasses, especially one or more of nitrogen, carbon dioxide, lower hydrocarbons and/or water, by contacting the gas mixture with a non-porous polyimide-based membrane, especially with the selective side of an asymmetric membrane, to obtain a hydrogen rich permeate and a hydrogen lean retentate. The invention especially concerns the use of specific blends of polyimides. Membranes made from these blends are especially suitable for the gas phase removal of hydrogen from a hydrogen and carbon monoxide containing gas mixture and show a particularly useful selectivity between hydrogen and carbon monoxide. The invention is especially suitable for the upgrading of synthesis gas obtained by partial oxidation and/or reforming of hydrocarbonaceous feedstream for use in synthesis gas conversion reactions as the Fischer-Tropsch reaction, methanol synthesis and dimethylether synthesis.
Polyimide membranes for gas phase separation are well known and are used in commercial applications such as the production of oxygen enriched air, the separation of carbon dioxide from methane and the separation of hydrogen from hydrocarbons. For certain gas streams one or more components may exhibit a strong interaction with the membrane material, which can plasticize the membrane. This holds especially for carbon dioxide. This may result in a detrimental decrease of the selectivity of the membrane.
At the present moment there is a strong interest in the production and use of synthesis gas obtained by partial oxidation and/or reforming of hydrocarbonaceous feedstocks as natural gas, biomass, heavy (residual) oil fractions and coal for the preparation of chemical compounds as methanol, dimethylether, ammonia, and, especially, paraffinic compounds, these paraffins especially suitable for use for the preparation of specialty normal and iso-paraffin mixtures and transportation fuels. The latter reaction, i.e. the synthesis of hydrocarbons, is well known in the art as the Fischer-Tropsch reaction. In general, synthesis gas is a mixture comprising mainly hydrogen and carbon monoxide. However, depending on the preparation process, it may contain considerable amounts of nitrogen, even up till 80 vol % of the total mixture, and carbon dioxide, even up till 40 vol % of the total mixture, usually up till 15 vol %. Further, several other components may be present as (unconverted) methane, water and one or more sulphur containing compounds.
A particular problem with the synthesis gas stream for use in especially the Fischer-Tropsch reaction is the H2/CO ratio. Depending on the gasification process and the starting hydrocarbonaceous feedstock synthesis gas with a specific H2/CO ratio is obtained. This H2/CO ratio is usually not the optimum ratio for the Fischer-Tropsch process. The Fischer-Tropsch process requires a specific overall H2/CO ratio (the so-called users ratio, usually 2.0 till 2.1), which requires specific measures to obtain this ratio. These measures could be: subjecting (part of) the syngas to a water-gas shift reaction; the combination of partial oxidation and steam reforming; additional production of hydrogen; import of hydrogen from a different source; the use of part of the CO for energy generation; etc. It is, in addition, also possible to adapt the H2/CO ratio by membrane separation of the synthesis gas in order to remove part of the hydrogen or part of the carbon monoxide. Such a process requires a highly selective and productive membrane. In the case of other syngas based synthesis reactions, similar problems as described above will apply.
Several processes are known to separate hydrogen from syngas. Cryogenic processes, for instance, are well known. However, such processes require high amounts of energy in order to create the low temperatures.
In US 2003/0223931 is disclosed a method for forming synthesis gas with a tailored hydrogen/carbon monoxide ratio, wherein the synthesis gas is subjected to a membrane separation process using a polyimide membrane. In Shishatskiy et al. “Polyimide Asymmetric Membranes for Hydrogen Separation Influence of Formation” Advanced Engineering materials, vol. 8 (2006), 390-397, the use of dense membranes of a specific polyimide (Matrimid 5218, a polyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diamino-phenylindane) for gas separations is described.